31 Dec
2021
31 Dec
'21
9:36 p.m.
I enabled user accounting on my v7 instance and I noticed it "growing
without bound" and while this is noted as a possibility in the ac(1) man
page, I was pretty sure the original authors didn't mean 30k a second. I
scratched my head and thought for a while and then started experimenting
to see what the heck was going on. I removed /usr/adm/wtmp (which I had
created to enable accounting in the first place) and the little red disk
write arrow on my mac went away, but not the little green disk read
arrow... hmm. Something was keeping my v7 instance very busy reading
disk, that was for sure. I went through a few (dozens) more tests and
experiments, reread a bunch of man pages, Ritchie's v7 install note, and
thought some more and here's what I came up with...
If you modify your system to add dci lines and you enable some ttys in
/etc/ttys and you enable user accounting. Then, the next time you boot
into a kernel that doesn't have dci support, init or some other process
will try and fail to read the enabled ttys, log something in
/usr/adm/wtmp, if it exists, and then loop (very quickly), over and over
and over. If you aren't paying attention, this will hardly be noticeable
on modern hardware running simh, but I'm guessing this would have been
disastrous, back in the day.
The simple solution is to boot w/dci enabled when you have ttys enabled,
and only boot w/o dci enabled when you have disabled the ttys.
I'm guessing that this wasn't really ever an issue, back in the day, as
folks prolly didn't just yank their dci's and reboot a different kernel?
But, such are the joys of simulation.
Anyhow, if this doesn't sound like a very likely or reasonable analysis
of what was happening, I'd appreciate your letting me know, or if you've
experienced something like it before, it'd be great to know that I'm not
alone in this silliness.
Will